1. Field of the Invention
This invention relates to electrochemical cells and more particularly, to a method of catalyzing electrochemical cell electrodes and the electrodes so produced.
2. Description of the Prior Art
A well-known and important type of electrochemical cell is a fuel cell which reacts a fuel and an oxidant at a pair of electrodes to make electricity. Low temperature fuel cells require catalysts in each of the electrodes to promote the reaction of the fuel and the oxidant. But, the electrochemical reaction of each of the reactant gases takes place in the presence of the catalyst only in those regions of an electrode in which the electrolyte and the reaction gas establish an interface and the electricity produced can be taken away. If there is catalyst at other places in the fuel cell electrode, or if catalyst is lost in processing, that catalyst is wasted. Catalysts typically used in fuel cells are expensive noble metals and therefore it is desirable to reduce the waste as much as possible and still have good fuel cell performance; that is, efficient utilization of the catalyst is essential. To achieve an electrode in an electrochemical cell that can provide a high current density and maintain a high voltage, it is necessary to have a large electrolyte/reactant gas interface area. It is known in the prior art that use of small and distinct hydrophobic areas through which the reactant gas can pass and hydrophilic areas in which the electrolyte can be present, allows for large interfaces. In one type of electrode having these characteristics which has found wide acceptance, catalyzed agglomerates of porous carbon particles are bound together with polytetrafluoroethylene (PTFE) to establish the hydrophobic and hydrophilic areas. One known method for catalyzing such electrodes applies a catalyst to the carbon particles before the carbon particles are bound together with the PTFE and put onto a current collector support to form an electrode: this is known as pre-catalyzation. The pre-catalyzation method deposits catalyst on all of the carbon particles that are to be used in the electrode and, as a result, some of the catalyst is wasted because some of it is placed where there may be no electrolyte/reactant gas interface or where there is no electrical path out of the cell. The precatalytic techniques also involve losses in the original treatment of the carbon as well as in the handling of the catalyzed carbon during fabrication of the electrode.
In addition, it has sometimes been observed that precatalyzed electrodes do not function as efficiently as electrodes which are fabricated according to "post-catalyzation techniques." A post-catalyzation technique is one in which the catalyst is deposited in the electrode structure after the electrode structure has been formed. Although resulting in superior performing electrodes all post-catalyzation techniques hitherto employed have been extremely difficult processes to control. This results in a substantial amount of the catalyst being deposited in areas where it is not desired. According to this invention, we have found a method by which post-catalyzation of conducting particle, hydrophobic bonded substrate supported electrodes can be obtained by simple and extremely controllable techniques.
It is, accordingly, an object of this invention to provide an efficiently catalyzed electrochemical cell electrode.
Another object of the present invention is to provide a method for post-catalyzing an electrochemical cell electrode.
It is another object of this invention to provide a method for pre-wetting, with an electrolyte, an electrochemical cell electrode.
It is another object of this invention to provide methods for selectively depositing a catalyst in a prewetted electrochemical cell electrode.